Newbie here.
I just have been using Linode this last few weeks, and I am still stuck on this thing.

I have 2 wordpress websites on my Linode (1 server), but I figured out that it doesn't look good to see my example1.com has a hostname of my test.com.
So I tried to find how to make my example1.com has a hostname of "example1.com", and my "test.com" has a hostname of "test.com", but after days of trying, there was nothing work.

There are a couple of things at play here. First lets assume you actually own 2 domains names.
For the sake of the exercise we will assume you own example.com (substitute your 1st real domain name here) and example.net (substitute your real 2nd domain name here)

OK, I went to check-host and ran a few domains through it. The answer is probably no.
The hostname is the name of your Linode server. Since you only have 1 Linode, you only have 1 hostname. It's kind of like your first name, you can have only 1 first name.

check-host is probably using a reverse DNS (RDNS) lookup to determine the hostname of your Linode at the IP address of the domain name you type in.

If that is the case, and you have 2 IP addresses, you may be able to set 2 reverse DNS entries for your server so that 10.10.10.1 points to Jupiter and 10.10.10.2 points to Mars, using the real IP addresses and domains of course.
You would then have to host your 2 websites on the different IP addresses, so check-host does the RDNS lookups of the different IP addresses and get different answers for the hostnames.

All this does is hide the fact you are running 2 websites on 1 server, which is not really a big deal in the scheme of things.

If you don't have 2 IP addresses, it's probably not possible.

Edit:
If you are getting the result lixxx.member.linode.com you probably haven't setup the RDNS entry in the Linode control panel.